Add support for JSONP callbacks on entity data feeds

Details

Description

To enable mashup scenarios, it should be possible to wrap a JSON serialization of an entity in a callback (JSONP). Common web APIs provide a default named callback but also allow specifying one as a GET parameter to facilitate usage of various scripting libraries. This functionality should be made available in the default distribution of the Entity Broker so that developers need not implement a custom format when JSONP is desired.